Prior to the new migrations of the fungus from Mexico, a clonal lineage, US1, predominated throughout the world.
Clonal propagation -- asexual reproduction, including vegetative spread, producing a clone of the parent plant.
Perhaps the drive for clonal diversity on resistant rootstock is due to new vineyard development and the replanting of infected vineyards.
The pattern of somatic mutation is being used to follow the clonal history of the tumor cell clone.